Actually, that's almost the exact behavior I've seen too. Hitting save and get the error, go back and I get ONE chance to make any corrections. If I get a 502 error a second time, it will clear all but the first two lines of the table and I have to start from scratch. And I recall making edits to switches when this happens too. I've got to the point that unless I've got a full hour or so to build a single table, I don't bother because I now expect it will take me a couple tries.